In networks, a plurality of routers relay a packet and transmit it to a destination computer. Each router exchanges route information with other routers in order to recognize transmission routes to the destination of the packet. The route information includes a transmission route which a router that sends the route information recognizes. Each router selects a packet transfer destination on the basis of the route information received from the other routers.
If there are a plurality of transmission routes from a router to a destination computer, then the router performs route selection in accordance with a predetermined algorithm. A route selection algorithm used by each router is provided in a routing protocol such as OSPF (Open Shortest Route First) or RIP (Routing Information Protocol). With these routing protocols, basically route selection is performed so that the shortest route will be taken. Alternatively, the existing advanced control using these routing protocols is exercised. That is to say, smoothing is performed so that the peak of network usage will be minimized. At the same time route selection is performed so that the shortest route will be taken.
If the above conventional routing protocols are used, traffic such as a packet is transmitted along a predetermined route. Accordingly, even when a traffic volume is low throughout a network, it is necessary to continue starting each router.
With the improvement of network throughput, on the other hand, the consumption of power by devices included in networks has increased. Therefore, the operation which controls the power consumption of an entire network is important. As a result, the following technique is proposed. A network controller monitors the state of traffic in a communication network. When a traffic volume is low, the traffic is transmitted along a detour. As a result, a part of routes is used and switching systems can be stopped. This procedure can save electric power (see Japanese Laid-open Patent Publication No. 2001-119730).
In some networks, however, it is difficult to exercise centralized traffic control by a management system. In a large-scale network, for example, it is difficult to exercise centralized control of all traffic by a management system. In addition, if centralized traffic control is exercised on each transmission route by a management system, the influence of trouble, such as a system malfunction, may spread throughout a network. This leads to a deterioration in the reliability of an entire system.